1. Field of the Invention
The present invention relates to an angular rate (an angular velocity) sensor and an acceleration sensor used, for example, in vehicle motion control, vehicle direction finding, etc.
2. Related Art of the Invention
Hitherto, various gyroscopes (hereinafter abbreviated as gyros) have been developed as sensors for detecting angular rate. They are roughly classified into the mechanical gimbaled spining wheel, fluid type gas rate gyroscope, vibratory gyroscope making use of vibration of tuning bar or tuning fork, fiber optic gyroscope, and ring laser gyro. The optical gyroscope detects the angular rate by making use of Sagnac effect, while others do by making use of Coriolis force which is an expression of angular momentum conservation law of a rotating body, and a proper sensor is selected depending on the purpose of use in consideration of precision, price, dimensions, etc.
A vibratory gyro type angular rate sensor is disclosed, for example, in Japanese Laid-open Patent No. 61-77712. FIG. 20 is an explanatory diagram of basic principle of this conventional vibratory angular rate sensor, in which 1401, 1402 are detecting elements, and 1403, 1404 are oscillating elements. Each element is composed of, for example, piezoelectric bimorph, and two sets of oscillating elements and detecting elements compose a tuning fork. The angular rate is detected by applying a alternating-current voltage to the oscillating element near the root of the tuning fork to vibrate the detecting element flexurally, and detecting the Coriolis force vertically applied to the surface of the detecting element by making use of the piezoelectric effect.
In the automotive application, it is often used in control of the chassis system, vehicle direction finding for navigation system. What is detected is, among three types of car body rotary motions of yaw, roll and pitch, often the angular rate in the yaw direction (rotation within a plane horizontal to the ground about the vertical line) (that is, yaw rate). The purpose of detection is, for example, in the case of chassis control of four-wheel steering (4WS) type, to improve the motion control performance by feeding back the yaw rate to the control system side as one of vehicle motion information, and in the case of navigation system, it is to calculate the turning angle of the vehicle by integrating the yaw rate by the time. The angular rate sensor for mobile use is generally the piezoelectric vibratory gyro and fiber optic gyro, and the optic gyro is already used for high precision application and the vibratory gyro, as inexpensive gyro, for mobile use.
As conventional examples of acceleration sensor, aside from the piezoelectric type reported, for example, by Asano et al. (Development of Acceleration Sensor and Acceleration Evaluation System for Super Low Range Frequency, SAE '91, pp. 37-49), the mechanical type and various types are being studied. As automotive applications, it is used in collision detection of air-bag and control of chassis system, and the magnitude of the acceleration to be detected is about .+-.50 G at maximum for air-bag and about .+-.2 G at maximum for chassis control. For mobile use, at the present, various techniques have been realized for both applications.
For mobile use or application in various devices, however, downsizing of electronic components is essential, and, for example, even the so-called small-sized piezoelectric vibratory gyro is not sufficiently small as compared with other electronic components, and further downsizing of angular rate sensor and acceleration sensor has been demanded.